1. Field of the Invention
The present invention relates in general to thermal ink jet (TIJ) printheads and more specifically to a system and method for high-performance printing that uses a compact monochrome printhead having staggered, high-density arrangement of ink drop generators.
2. Related Art
Thermal ink jet (TIJ) printers are popular and widely used in the computer field. These printers are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Ink jet printers produce high-quality print, are compact and portable, and print quickly and quietly because only ink strikes a print medium (such as paper).
An ink jet printer produces a printed image by printing a pattern of individual dots (or pixels) at specific defined locations of an array. These dot locations, which are conveniently visualized as being small dots in a rectilinear array, are defined by the pattern being printed. The printing operation, therefore, can be pictured as the filling of a pattern of dot locations with dots of ink.
Ink jet printers print dots by ejecting a small volume of ink onto the print medium. An ink supply device, such as an ink reservoir, supplies ink to the ink drop generators. The ink drop generators are controlled by a microprocessor or other controller and eject ink drops at appropriate times upon command by the microprocessor. The timing of ink drop ejections generally corresponds to the pixel pattern of the image being printed.
In general, the ink drop generators eject ink drops through an orifice (such as a nozzle) by rapidly heating a small volume of ink located within a vaporization or firing chamber. The vaporization of the ink drops typically is accomplished using an electric heater, such as a small thin-film (or firing) resistor. Ejection of an ink drop is achieved by passing an electric current through a selected firing resistor to superheat a thin layer of ink located within a selected firing chamber. This superheating causes an explosive vaporization of the thin layer of ink and an ink drop ejection through an associated nozzle of the printhead.
Ink drop ejections are positioned on the print medium by a moving carriage assembly that supports a printhead assembly containing the ink drop generators. The carriage assembly traverses over the print medium surface and positions the printhead assembly depending on the pattern being printed. The carriage assembly imparts relative motion between the printhead assembly and the print medium along a “scan axis”. In general, the scan axis is in a direction parallel to the width of the print medium and a single “scan” of the carriage assembly means that the carriage assembly displaces the printhead assembly once across approximately the width of the print medium. Between scans, the print medium is typically advanced relative to the printhead along a “media (or paper) advance axis” that is perpendicular to the scan axis (and generally along the length of the print medium).
As the printhead assembly is moved along the scan axis a swath of intermittent lines is generated. The superposition of these intermittent lines creates the appearance as text or image of a printed image. Print resolution along the media advance axis is often referred to as a density of these intermittent lines along the media advance axis. Thus, the higher the density of the intermittent lines in the media advance axis the greater the print resolution along that axis.
The density of the intermittent lines along the media advance axis (and thus the print resolution) can be increased by increasing the number of ink drop generators on the printhead. This results in better print resolution and increased print speed. Moreover, due to several factors, it is desirable to increase the number of ink drop generators without increasing the size of the printhead. However, merely increasing the number of drop generators on an existing printhead greatly increases the amount of heat dissipated in the printhead during print operations. This increased heat dissipation can cause unwanted printhead thermal excursions. These large thermal excursions on the printhead adversely affect printhead operation and can cause print quality defects, printhead thermal shutdown and even failure of the entire printhead.
One technique that may be used to avoid large thermal excursions is to slow down the speed of the printhead. This technique, however, negates the positive effect of providing more ink drop generators on the printhead. Another technique that may be used to avoid a large thermal excursion is to increase the size of the printhead. A primary disadvantage of this technique, however, is that increasing printhead size increases the cost of the printing system. This is unacceptable because printing systems are rapidly decreasing in price and a printing system having the added cost of a larger printhead will not be competitive in the marketplace. What is needed, therefore, is a way of providing a compact, high nozzle count, and high performance printhead that does not suffer from deleterious thermal excursions.